The purpose in this application is directed towards understanding the mechanisms which cause regression of the corpus luteum, a gland in the ovary responsible for synthesis and secretion of the hormone progesterone. The vital role of progesterone in inhibition of the pituitary hormone secretion necessary for ovulation and the requirement of this steroid hormone for development of the endometrium to receive the fertilized egg indicates the importance of the corpus luteum in reproduction. An understanding of the factors which control regression may lead to utilization of this information for interupting luteal function to prevent an unwanted pregnancy or lead to therapy to maintain a threatened pregnancy. The research in this proposal is focused upon the role of prostaglandins and the mechanism whereby they induce luteal regression. In particular, the basis of the antigonadotropic action of prostaglandins is studied in conjunction with studies on the mechanism of gonadotropic hormone action. Thiss focus arose from observations which clearly show that functional activity of the corpus luteum is dependent upon gonadotropins and loss of action of gonadotropic hormones leads to a rapid loss of function. To study those problems, changes in gonadotropin receptor binding activity, actions of the receptor - hormone complex in vivo and in vitro, ovarian blood flow and uptake of radiolabelled hormone by luteal tissue in vivo will be examined.